The creation of integrated optical devices from separate micro-components has, in the past, required time-consuming and often manually intensive methods. Attempts to alleviate these difficulties have seen the emergence of more mechanized technologies that focus on assembly either via fluidic self-assembly or methods that are based on wafer-to-wafer transfer. Key to all these technologies is the substrate which is either a specifically prepared ‘receptor’ with precisely etched holes that are complementary to the optical components, or substrates that require equally stringent photolithographic alignment and/or masking. The current technologies used for the integration of optical components are restricted by the limited number of compatible substrates (e.g. silicon, silicon oxide, gallium arsenide).
Ideally, the optical designer should not be limited by the fabrication technology. For example, one should be able to integrate III-V light sources and detectors with Si based photonic crystals, modulators and/or micro-mirrors, with SiO2 waveguides, and non-linear optical devices on any substrate. The function and/or complexity of an integrated optical circuit should not be restricted by the substrate.
“Strained layer epitaxy” is used to integrate semiconductors with dissimilar lattice structures, such as growing GaAs on Si, or SiGe alloys on Si, etc. However, this technique is only possible if the respective layer thicknesses are thinner than a critical thickness which is typically extremely thin. In addition, this technique is only useful for crystalline materials, and is not useful for integrating non-crystalline materials such as plastics and glasses. The use of MEMS (Micro-Electro-Mechanical Systems) for integrating mechanical components, sensors, etc with electronics on a silicon substrate using microelectronic technology is also made use of. This technology relies on devices, such as micro-mirrors, waveguides, cantilevers, etc that are Si (and SiO2) based and are micromachined into Si. Again, this method is limited to Si and SiO2 and is not useful to integrate other materials, such as GaAs, electro-optic materials, etc
There are a number of other techniques that are grouped into ‘top-down’ and ‘bottom-up’ approaches. The top-down approach involves a block of material being processed into the desired shape and working unit. In bottom-up fabrication, small building blocks (usually nanoscale as the term originates from nanotechnology) are connected together to fabricate a functioning unit.
Current top-down approaches for integrating optical structures on a substrate typically involve fluidic assembly into defined ‘holes’ in a substrate, lithographic patterning followed by etching or wafer-to-wafer transfer. These are very complicated procedures that lack the ability to be easily scaled up and typically suffer from low fabrication success rates.
On the other hand, while there are many potential bottom-up strategies for fabricating optical structures on different materials, no current method for assembling high quality optical devices (prefabricated) on any substrate has been demonstrated. A sufficient understanding of how to assemble molecular building blocks with sufficient control to produce high quality materials (that is, comparable to microelectronics state of the art) has not been reached.
Recently, methods for electric field assisted self-assembly of functionalized DNA strands as building blocks for assembly and fabrication of devices have been proposed in U.S. Pat. No. 6,652,808. However, the methods disclosed in that document focus primarily on the control and chemical nature of the DNA based building blocks for bonding of components to a substrate, rather than providing any teaching with respect to the properties or functionality of the devices bound to the substrate. Furthermore, an approach for building a photonic band-gap structure is disclosed, where a photonic band-gap structure is built-up from metal beads exhibiting magnetic properties. The photonic band-gap structure is formed on the substrate through a process in which the metal beads are interconnected via DNA bonds. No optical characterization of such grown photonic band-gap structures is provided in that document.
Furthermore, there is no teaching provided in that document that verifies whether the alignment accuracy between the metal beads is actually sufficient to achieve a photonic crystal effect, and on which substrate or type of substrates. A technique for alignment of “larger” structures of the order of 10 to 100 microns is also discussed in that document, using selective derivatisation with different DNA sequences of a device to be positioned and oriented on a substrate. However, no teaching is provided with respect to handling of larger devices, thus limiting the proposed method to techniques in which the devices to be attached are smaller than about 100 microns, and with a need to apply individual devices in that size range to the substrate for assembly. The preparation of free-standing devices in that range of small sizes can constitute a major challenge in the overall assembly process, in particular with a view to mass-production of assemblies of devices on various substrates.
As an example application of integrated optical devices, currently, optical methods for sensing molecular species often require a sample cleanup, where the target analyte resides in a complex mixture of many different molecules. Many current optical methods also require the labeling of the analyte using for example, a fluorescent tag, and complex instrumentation that requires both transport of the sample to a laboratory and trained personnel. The prior art optical methods also require time-consuming protocols with long incubation periods, wash steps etc. The combination of these factors will often lead to the slow detection of a chemical or a biological molecule. However, in many situations, expediency is integral in detecting a substance for example, at times of environmental threat, point-of-care diagnosis, biological and chemical warfare. Hence, many prior art sensing technologies are inadequate. Although there are currently a number of label-free methods for sensing molecular species, these methods suffer from either non-specific detection issues, poor sensitivity compared to labeling approaches, incompatible formats for the field or other disadvantages such as complicated instrumentation, the need for skilled technicians or the need for sample cleanup or a combination of the above.
Photonic crystals formed by electrochemical etching porous silicon (PSi) are an example of ‘hard’ photonic crystals that can be fabricated by modulating the porosity and hence the refractive index of the layers during anodization [A. G. Cullis, L. T. Canham, P. D. J. Calcott, Applied Physics Reviews 1997, 82, 909.] The nanoporous architecture of the PSi material allows infiltration of gases and liquids within the material, thus modifying the average refractive index and the resultant spectral qualities. This quality of PSi materials has led to numerous investigations of PSi materials in optical sensing including gas, chemical and biological sensing. [M. P. Stewart, J. M. Buriak, Adv. Mater. (Weinheim, Ger.) FIELD Full Journal Title: Advanced Materials (Weinheim, Germany) 2000, 12, 859.; S. D'Auria, M. de Champdore, V. Aurilia, A. Parracino, M. Staiano, A. Vitale, M. Rossi, I. Rea, L. Rotiroti, A. M. Rossi, S. Borini, I. Rendina, L. De Stefano, J. Phys.: Condens. Matter FIELD Full Journal Title: Journal of Physics: Condensed Matter 2006, 18, S2019.; G. Marsh, Mater. Today (Oxford, U. K.) FIELD Full Journal Title: Materials Today (Oxford, United Kingdom) 2002, 5, 36.; T. Islam, H. Saha, Sens. Actuators, A FIELD Full Journal Title: Sensors and Actuators, A: Physical 2007, A133, 472.]
One type of PSi photonic crystal that has shown utility for sensing is the resonant microcavity. [P. J. Reece, M. Gal, H. H. Tan, C. Jagadish, Applied Physics Letters 2004, 85, 3363.; L. Rotiroti, L. D. Stefano, I. Rendina, L. Moretti, A. M. Rossi, A. Piccolo, Biosensors & Bioelectronics 2005, 20, 2136.; L. D. Stefano, I. Rea, I. Rendina, L. Rotiroti, M. Rossi, S. D'Auria, Physica Status Solidi A: Applications and Materials Science 2006, 203, 886.; L. A. DeLouise, B. L. Miller, Analytical Chemistry 2004, 76, 6915.; L. A. DeLouise, B. L. Miller, Analytical Chemistry 2005, 77, 1950.; L. A. DeLouise, P. M. Kou, B. L. Miller, Analytical Chemistry 2005, 77, 3222.; H. Ouyang, M. Christophersen, R. Viard, B. L. Miller, P. M. Fauchet, Advanced Functional Materials 2005, 15, 1851.; H. Ouyang, L. A. DeLouise, B. L. Miller, P. M. Fauchet, Analytical Chemistry 2007, 79, 1502.; H. Ouyang, C. C. Striemer, P. M. Fauchet, Applied Physics Letters 2006, 88, 163108.]. Microcavities are formed by incorporating a defect (spacer) layer within the periodicity of a multilayered 1-dimensional photonic crystal stack. Tuning the optical thickness (n·d, where n is the refractive index and d the thickness of the layer) of the spacer layer to mλ/2 (λ is the central wavelength of the Bragg plateau, m is the spectral order) gives rise to a cavity resonance in the centre of the spectrum, where light of that wavelength “resonates” and therefore does not reflect.
In the prior arts using PSi microcavities for sensing stimuli such as biomolecules or chemicals etc., the infiltration of material can cause shifts in the entire spectrum that can be correlated to the influx of material throughout the nanoporous matrix. Another drawback to using microcavities for sensing in existing sensor designs associated with the requirement that stimuli must reach the central layer is that the stimuli will need to penetrate from the top layer of the micro cavity through the nanoporous architecture, a particular problem for large biomolecules (comparable to or larger than the smallest pore size in the alternating pore size multi layered stack pore size). Attempts to alleviate this problem have included enlarging the pore diameter which leads to decreased optical quality and sensitivity. [H. Ouyang, C. C. Striemer, P. M. Fauchet, Applied Physics Letters 2006, 88, 163108.] Other attempts to address this problem have included modifying the surface chemistry within the nanoporous matrix which may enhance the ingress of particular species, the diffusion issue is still not solved. Hence, the modification of surface chemistry may allow excellent control over the type of analyte captured but its use is still limited by the diffusion issue.
As another example application of integrated optical devices, currently, there is a research interest into fabricating Si integrated optical epitaxial light emitting structures for optoelectronic technologies. While II-VI quantum dot doped microcavities have been reported for TiO2—SiO2 distributed Bragg reflectors have been reported e.g. in [L Guo, T D Krauss, C B Poitras, M Lipson, X Teng and H Yang, Applied Physics Letters 89, 061104 (2006)], and ion doped porous Si microcavities e.g. in [H A Lopez and P M Fauchet, Applied Physics Letters 77, number 23, 4 Dec. 2000], the applicant is not aware of reports on quantum dot doped microcavities formed using Si integrated optical epitaxial techniques.
The present invention has been made in view of the above described background to seek to address one or more of the above-mentioned problems.